Quantitative trait loci (QTLs) are chromosomal regions containing genes that influence a complex trait such as drug withdrawal severity. We have established that there is a great deal of common genetic influence on withdrawal from barbiturates, benzodiazepines, nitrous oxide, and alcohol. During the current period, we have mapped several QTLs that jointly have a major influence on the severity of pentobarbital (PB) withdrawal. The three largest QTLs are on mouse chrs 1, 4 and 11. QTLs in each of these regions have also been provisionally mapped for diazepam withdrawal, and definitively mapped for ethanol withdrawal: the chr 1 QTL was also provisionally mapped for nitrous oxide withdrawal. Using congenic strains to isolate each of the three QTLs against a uniform (inbred) genetic background, we propose to continue toward the eventual identification of the genes that underlie each PB withdrawal QTL. We propose to: (1) Test congenics for the strongest PB QTLs for their pleiotropic effects on withdrawal from other drugs of abuse; (2) Produce polycongenics in different combinations to determine whether gene-gene (epistatic) interactions are additive, potentiating in some combinations, or epistatic in some other way; (3) Narrow each QTL interval from our present approximately 20 cM to approximately 1 cM using interval specific congenic strains (ISCS); (4) Scan promising candidate genes for cDNA differences between B6 and D2 genotypes by SSCP; (5) Produce polycongenics from specific donor segment (SDS) congenics produced from appropriate interval specific congenic strains, and test for epistatic interactions among QTLs, as well as QTL pleiotropisms for withdrawal from other drugs, and for other drug-related responses known to be genetically correlated with PB withdrawal severity; and (6) Start with an F2 population, screen for additional QTLs not previously ascertained and produce additional congenics to facilitate eventual cloning of the genes.